The fallacy of knowledge aka “under” standing

Today I was watching a video by 3BlueBrown describing how something with probability zero need not be an impossible event.
While watching the video, I “understood” things quite well, but when I tried to explain the same to my friend, I wasn’t able to do so and I ran into much confusion.
Then I remembered what Einstein used to say, if you can’t explain something to a 6 year old kid and make him understand it, then you have not really understood that topic !